I was looking at this question and others similar, but I'm a bit unsure if the same applies for my case: I recently made a site which cuts for browser support at IE9. However the old site, which mine replaces, does have support all the way back to IE6.
As the content is fairly similar on both sides I'm a bit worried about getting punished for duplicated content. For now I've disallowed the IE site for indexing altogether. But if there is something to gain of allowing indexing of both sites, that would of course be best.